FACTS ABOUT MOUNTAIN REGIONS. 



793 



Fractures may cut through the rocks vertically. But they usually 

 have more or less obliquity — frequently as much as 45°, and some- 

 times a dip of only 25° or 30°. The case in the Alps, here repre- 



Figs. 1146-1150. 



1146. 



1147. 



1148. 



1149. 



1150. 



Folds passing into Faults, 

 sented, is one of very nearly 45°. The fissures occupied by the 

 trap of the Connecticut Valley, near New Haven, Conn., vary in dip 

 from 30° to 70° ; in one north-and-south dike, 300 or 400 feet wide, 

 it averages 30°; in another, 45°; in two east-and-west dikes, each 

 about 200 feet wide, 50° and 67°. 



The fracturing agency has produced widely unlike effects on differ- 

 ent kinds of rocks. If hard and comparatively unyielding, long, even 

 fissures are often made ; if brittle, like a limestone and some sand- 

 stones, they have been very irregular and the rock much broken ; 

 while sand-beds that were not firm have had the fractures obscured by 

 compression, or a rearrangement, and clayey beds have been stretched 

 as well as compressed. The fracturings, as seen at the surface, may 

 be very various in direction and variously subdivided, owing to the 

 irregular breakings and faul tings of the surface rocks, when in the 

 crystalline rocks beneath, and in the earth's crust, they were very 

 regular, and for long distances uninterrupted. 



3. Jointed Structure. — Systems of joints (explained and illustrated 

 on page 88) belong especially to the rocks of disturbed regions. 

 Slaty cleavage is a variety of it, occurring in fine-grained rocks. It 

 has no reference to the bedding, being transverse to it, or parallel to 

 it; but the latter, only when the beds stand at a high angle. In 

 granite, vertical joints are very common. But a horizontal cleavage 

 is often quite distinct ; and it is sometimes so decided in granite-like 

 gneiss that were it not for the existence of undoubted nearly vertical 

 lines of bedding in the gneiss, the horizontal planes would be taken 



